Current commercial CMOS imagers are front-illuminated. FIG. 1 shows a vertical cross-section of the optical collection part of a front-illuminated pixel.
The photodetector 10 comprises an ion-implanted cathode 20 on an epitaxial or substrate silicon layer 30 that acts as the anode. The photodetector 10 is mechanically supported by a thick (about 0.5 to 0.7 mm) silicon substrate 40, in keeping with conventional VLSI micro-fabrication paradigm.
FIG. 1 also shows metal layers 50 for interconnection of circuits and photo-detectors fabricated on the epitaxial silicon layer 30. The metal layers 50 are separated and protected by inter-layer dielectric (ILD) 60.
The main problem of imaging with a structure as shown schematically in FIG. 1 is the increased distance between the point 70 where light enters the system and the silicon 30 where light is detected, i.e. converted to photoelectrons. As shown in FIG. 1, light has to travel trough many layers of dielectric and interconnect metal layers (metal bus lines) suffering multiple reflections, obscurations, and deflections, before it is actually collected by silicon. For a small sized pixel, the aspect ration between the vertical distance to the photodiode width can be as high as 3:1. This is akin to shining flash light in a canyon. Due to the increased distance between the color-filter/micro-lens and the silicon surface, the device suffers from poor collection efficiency, low sensitivity, low quantum efficiency (QE), increased cross-talk, and poor angular response.
QE loss occurs due to a loss of optical fill factor (defined as the ratio of the optical collection area to the pixel area), especially as the pixel size is scaled. Poor angular response results from the increase in the aspect ratio, especially as the pixel size is scaled down as well as from increased unwanted reflections and occultations at metal edges. Increased cross-talk is due to a large separation (determined by the ILD thickness) between the silicon and color-filter layers, and due to lateral movement of focus point as the angle of acceptance is changed.
In other words, the front-side illumination structure of FIG. 1 suffers from poor QE and angular response uniformity, increased optical cross-talk, and stray-light coupling, especially as the pixel size is scaled. The addition of an anti-reflection coating is nearly impossible because of the presence of multi-layers with unfavorable dielectric constants and due to non-planarity of the photo-collection junction.
Technology scaling actually makes the problem worse, since the number of metals, and the thickness of ILDs increases with scaling, resulting in an even higher skewing of the aspect ratio. Furthermore, the introduction of low-k dielectric, and use of alternate metals (e.g. Cu) for interconnection is expected to further exacerbate the problems through increased absorption and scattering in the metal-dielectric stack.